Genetic Studies of the Campylobacter rectus Toxin. Campylobacter rectus is a Gram-negative, motile, anaerobic, rod-shaped bacterium frequently isolated from adult periodontitis lesions. One potential C.rectus virulence factor is a toxin which kills human polymorphonuclear leukocytes and monocytes. Previously, we cloned a region from C.rectus that reacted with two probes generated fom the N-terminal sequences of the 104 kDa toxin and with an A. actinomycetemcomitans leukotoxin probe. In an expression vector, the 6 kb insert expressed a 60 kDa protein reactive with polyclonal antibody to the 104 Kda protein. In view of the truncated nature of this protein, we developed C.rectus phage library. Whole genomic DNA from C.rectus was partially digested with the restriction endonuclease Sua 3A and ligated into the Bam HI site of lambda GEM-12 vector. The library was screened with labeled insert from the positive clone and with the anti-toxin antibody resulting in three positive clones. Sequence analysis of the plasmid clone indicated significant sequence similarity between the cloned C. rectus toxin and other cytolysins including the Escherichia Coli hly gene (48.9%), Pasteurella haemolytica ltx gene (47.7%), and the A. actinomycetemcomitans ltx gene (42.5%). Based on our sequence data we conclude that the C. rectus toxin may be closely related to the A. actinomycetecomitans leukotoxin and similarly may have an important role in the pathogenesis of certain forms of periodontal disease.